Re: [xhr] Remove XMLHttpRequest from ServiceWorkerGlobalScope (#19)

Actually `ProgressEvent` is exposed only on `Window` in Blink, but I believe that's an oversight. I think the spec should expose all the interfaces in the same contexts and Blink should change.

To me, throwing `InvalidAccessError` when sync XHR is attempted in any non-whitelisted context sounds pretty good. Which kinds of workers already depend on sync XHR I don't know, but we could measure it.

@coonsta and @jakearchibald, thoughts?

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